The present invention relates to a thermal spray powder. In particular, the invention relates to molybdenum-based thermal spray powders useful for producing wear resistant coatings on the sliding contact friction surfaces of machine parts such as piston rings, cylinder liners, paper mill rolls, and gear boxes.
Thermally sprayed molybdenum coatings, due to their unique tribological properties, are useful in the automotive, aerospace, pulp and paper, and plastics processing industries. Molybdenum coatings provide a low friction surface and resistance to scuffing under sliding contact conditions.
Coatings which are flame sprayed from molybdenum wire sources are widely used in the automotive industry as, e.g., running surfaces on piston rings in internal combustion engines. The high hardness of these coatings is attributable to the formation during spraying of MoO.sub.2 which acts as a dispersion strengthener. However, the process of flame spraying coatings from molybdenum wire is not sufficiently versatile for the more complex applications being developed for molybdenum coatings. Some of these applications require higher combustion pressures and temperatures, turbocharging, and increased component durability. The molybdenum wire produced coatings do not meet these requirements. Further, there is an increasing need for the tailoring of coating properties based on periodically changing design requirements. Powder based coating technologies, e.g., plasma powder spray offer flexibility in tailoring material/coating properties through compensational control, which is not readily achievable using wire feedstock.
Coatings which are plasma sprayed from molybdenum powder are more versatile than coatings from wire, but are relatively soft, and do not exhibit adequate breakout and wear resistance for the automotive and other applications described above. The molybdenum tends to oxidize during spraying, leading to weak interfaces among the lamellae of the coating and to delamination wear. Also, the aqueous corrosion characteristics of molybdenum coatings are poor.
The molybdenum powder may be blended with a nickel-based self-fluxing alloy powder, for example, powder including nickel, chromium, iron, boron, and silicon, to form a Mo/NiCrFeBSi dual phase powder (also referred to in the art as a pseudo alloy). The improved wear characteristics of a coating flame sprayed from the blend result in a wear resistant coating with desirable low friction properties and scuff resistance.
When this pseudo-alloy powder blend is plasma sprayed, however, the molybdenum particles and the NiCrFeBSi particles tend to form discrete islands in the coating. Although the overall hardness is greater, in microscopic scale the molybdenum islands are still soft and are prone to breakout and failure. Once the wear process is initiated, the coating exhibits rapid degradation with increased friction coefficient, particle pull out, and delamination.
Another improvement in plasma sprayed molybdenum coatings is described in the publication by S. Sampath et al., "Microstructure and Properties of Plasma-Sprayed Mo-Mo.sub.2 C Composites" (J. Thermal Spray Technology 3 (3), September 1994, pp. 282-288), the disclosure of which is incorporated herein by reference. A dispersion strengthened coating is plasma sprayed from a Mo--Mo.sub.2 C composite powder. The Mo.sub.2 C particles dispersed in the molybdenum increase the hardness of the coating. Also, the carbon acts as a sacrificial oxygen getter, reducing the formation of oxide scales between molybdenum lamellae of the coating during spraying and decreasing delamination of the coating. However, the hardness, wear resistance, and aqueous corrosion resistance of the coating is not sufficient for some applications.
Further improvement in plasma sprayed molybdenum coatings is described in above-referenced application Ser. No. 08/390,732. The dual phase powder blend disclosed in application Ser. No. 08/390,732 adds NiCrFeBSi powder to the above-described Mo--Mo.sub.2 C composite powder. The coating made from this powder blend exhibits discrete islands similar to those described above for the Mo--NiCrFeBSi coating. The NiCrFeBSi islands have similar advantageous properties to those described above; however, the Mo.sub.2 C particles dispersed in the molybdenum increase the hardness of the molybdenum islands, slowing degradation of the coating. Also, the carbon acts as a sacrificial oxygen getter, reducing the formation of oxide scales on the molybdenum islands of the coating during spraying and decreasing delamination of the coating, as described above. However, the aqueous corrosion resistance and/or hardness of the coating are still not sufficient for some applications.
The present invention is directed to even further improving the properties of molybdenum coatings, whether they are plasma sprayed or flame sprayed.